Welding is a manufacturing or fabrication process that bonds materials, usually metals or thermoplastics, by causing coalescence—the process by which two separate units grow together, fuse, or merge into a single body. The materials are joined by liquefying or plasticizing (e.g., soften without liquefying) the areas to be bonded together, generally through the application of heat and/or pressure over time, promoting coalescence of the liquefied or plasticized material, and allowing the coalesced material to cool, thereby solidifying the bond. Welding can be used, for example, to join two or more workpieces or for operating on a single workpiece (i.e., to repair a crack or join a member.)
The quality of a weld is predominantly determined by its strength and the strength of the material around it. Weld quality is influenced by various factors, the most influential factor being the method of welding.
One such method is friction welding. Traditional friction welding, which is a form of solid-state welding, involves the rubbing of two workpieces together at a controlled speed to create mechanical friction. The friction generates heat that allows both components to reach a plastic state. Once a plasticized region is created, the workpieces are forced together to form a bond. The bond is initiated when layers of plasticized material from both components intertwine and create new layers of combined material. The bond is finalized by stopping the relative movement of the workpieces and allowing the plasticized region to solidify, thereby joining the pieces.
Friction stir welding, a species of traditional friction welding and solid-state joining techniques, combines the processes of extruding and forging. Friction stir welding uses a wear resistant, cylindrical shouldered tool with a profiled pin. Frictional heat is generated between two or more adjacent workpieces by slowly forcing the welding tool into the joint line between the workpieces and contemporaneously rotating the tool. The tool is fed into, and translated along the joint line between the two work regions, which are butted together, at a constant traverse rate. The frictional heat causes the workpieces to yield and soften without actually reaching the material's melting point. As it does so, the plasticized material is transferred from the leading edge of the tool to the trailing edge of the tool shoulder and pin, leaving a solid phase bond between the two workpieces.
Friction stir spot welding is a variation of the friction stir welding solid-state technique. To that regard, a friction stir spot weld is produced by the application of pressure and heat that is generated by friction between a rotating, wear-resistant profiled tool and the workpiece(s). However, unlike standard friction stir welding, the weld tool in friction stir spot welding does not traverse along the joint line of the workpieces, but is rather plunged into the workpiece(s) and contemporaneously rotated to produce a single spot weld.
The strength of a friction stir spot weld or joint is predominantly influenced by the plunge depth of the weld tool or the remaining thickness of the weld. Current friction stir spot weld machines utilize one of two methods to reach plunge depth: (1) Force and Time-Controlled, i.e., welding at a certain force level for a specified time interval; and (2) Time-Controlled, e.g., welding at a programmed plunge depth with a programmed plunge speed. With the addition of a given plunge speed, a weld time is calculated from the desired plunge depth, at the end of which, welding is ended. However, in these traditional friction stir spot weld methods, a “plunge depth” that is not a true depth of plunge is programmed. In this invention, plunge distance is used instead of the conventional plunge depth. Plunge distance is defined as the distance the weld tool is plunged relative to a point of reference on its rotational axis. Bottom thickness of a friction stir spot weld is defined as the remaining thickness at the bottom of the hole left by the weld tool (dimension C in FIG. 4).